1. Field of the Invention
The present invention relates to the field of insertable or implantable materials or devices in which the material or device is secured into the tissue of a patient through a helical or screw element which is secured into tissue or the like. In particular, the present invention relates to protective elements such as protective caps over a penetrating or pointed section of the material or device, wherein the protective element is capable of timely removal (as by dissolution) from the penetrating or pointed section during technical (e.g., medical) procedures. Typical devices for use in the present invention are connectors or leads for electrical stimulation or pacing of organs, such as cardiac pacers or defibrillators.
2. Background of the Art
Many therapeutic or protective procedures for patients include the implantation of devices into a patient. Such implantations include drug delivery systems, electrostimulating devices (such as pacemakers or pain reduction devices), monitoring devices, electrical leads, electrodes, sensor elements, etc. These devices often have to be firmly secured within the patient to prevent movement of the device that would defeat or diminish its effectiveness. This is particularly true with electrical leads in pacing or defibrillation devices, which must be precisely located so that electrical stimulation is effective. There are a number of different formats for the securement of electrical leads in patients, including, but not limited to, clips, sutured attachment, corkscrew-like inserts (referred to as helical inserts), and other conventional securement formats found in mechanical systems.
A preferred means of securing leads is the helical insert such as found in the GUIDANT(trademark) Sweet-Tip(trademark) Model 4269 bipolar endocardial lead. This lead comprises a helical element having a base side (proximal end) with an electrode and a sharp tip on an insert side (a distal end) of the element. The pointed end penetrates tissue when a rotating motion is applied to the helical element, causing the element to puncture and or screw into the tissue, advancing the proximal end towards the tissue. The proximal end may have a relatively flat or convex electrical plate, electrode, sensing element (e.g., semiconductor, circuit board, pressure plate, etc.) or contact, and the advancing of the helical element into the tissue brings the contact into firm position with the tissue. In pacing or defibrillating devices, the electrical discharge passes through the electrode and/or into the helical connecting element. In some leads, the helical element is coated with an insulating polymer (which must also be biocompatible) to render the helical element inactive or passive (from the standpoint of discharge). Typical polymer coatings could include polyamides, polyurethanes, silicone resins, polyacrylates, hardened gelatin, and especially poly-para-xylylene (e.g., Parylene C).
U.S. Pat. No. 5,964,794 describes an implantable stimulation electrode for use with an implantable tissue stimulator, in particular a pacemaker, defibrillator, bone or neurostimulator, having a metal substrate body and a coating, applied to the substrate body, for reducing the electrode impedance and/or increasing the tissue comparability, in which an ultrathin, specifically functionalized organic coating forming the entire outer surface of the stimulation electrode is provided, which adheres to the underlying surface as a consequence of irreversible physisorption or covalent chemical bonding. An organic layer is provided on the surface of an implantable stimulation electrode, which layer prevents or at least decisively reduces the nonspecific adsorption of biological macromolecules and is selectively specifically functionalized or functionalizable. Such an effect, which leads to a novel quality of biocompatibility while simultaneously obtaining high phase-boundary capacitance and hence low electrode impedance, is unattainable with the known stimulation electrodes having a metallic or inorganic surface. The term xe2x80x9corganic layerxe2x80x9d will be used hereinafter to include such a layer having silicon atoms, of the kind that can be formed by reaction with silanes, for instance. An additional functionalization of potential practical significance is that the organic layer has sensor molecules (such as enzymes) such that the stimulation electrode can act as a biosensor electrode. In a further important functionalization, the organic layer has a medicinal active ingredient, in particular an anti-inflammatory medication, which can be exported from the organic layer under diffusion or solution control. In particular, the medicinal active ingredient may be substantially embedded between constituent layers of the multilayer structure. The organic layer is ultrathin; that is, its layer thickness of the organic coating is in the range between 1 and 200 nm, and in certain versions (for instance as a polyelectrolyte multilayer) is preferably in the range between 3 and 50 nm. To assure advantageous electrical properties, and especially little influence on the high phase-boundary capacitance of highly sophisticated stimulation electrodes, even at relatively high layer thicknesses in the aforementioned range, the organic layer in an advantageous embodiment is embodied such that it has a relative dielectric constant of greater than 100 and in particular greater than 300. At very slight layer thicknesses, layers with a relatively low dielectric constant can also be used.
U.S. Pat. No. 5,080,099 describes skin electrodes with hydrogel contact elements as stimulation electrodes for an external defibrillator and/or pacemaker.
A suitable conductive gel 106 would be, for example, an RG 63T hydrogel.
U.S. Pat. No. 5,951,597 describes a coronary sinus lead having an expandable matrix anchor. An intravenous lead for use with a cardiac device for implantation in the coronary venous system of the heart includes a lead body that is adapted to be routed through the vascular system into the coronary sinus with the distal end portion of the lead placed in the great cardiac vein or branch vein. The lead body includes a fixation member disposed just proximal of its tip. The fixation member comprises a radially expandable polymeric matrix that incorporates an osmotic agent so that when placed in a aqueous medium it will swell. Thus, when placed in a cardiac vein, the swelling of the fixation member will anchor the lead against longitudinal displacement due to body motion, blood flow and the beating action of the heart.
U.S. Pat. No. 4,347,198 describes the manufacture of contact lenses where a hydrophilic component, for example N-vinylpyrrolidone, a hydrophobic component, for example methyl methacrylate, a cross-linking agent and an initiator are mixed in a solvent, for example DMSO, and then the whole is cross-linked in a mold. After extraction and equilibration in water, a soft hydrogel contact lens is obtained. Extraction with water is necessary because the solvent and unreacted vinyl monomers have to be removed. Since a polymer swells to different extents, for example in DMSO on the one hand and water on the other, the contact lens assumes its final size only at that stage.
EP 216 074 describes a process for the preparation of hydrogel contact lenses. There, a methacrylate-modified polyvinyl alcohol is used which is copolymerised in DMSO solution with vinyl monomers in a suitable casting mold, for example in the presence of a photoinitiator by irradiation with UV light for approximately 3 hours. After being removed from the mold, the contact lens is extracted with water or physiological saline solution in order to remove the DMSO and unreacted vinyl monomers. In this case too, the contact lens does not receive its final geometry until the final stage owing to the different influences of DMSO and water on its swelling behavior.
U.S. Pat. No. 5,931,862 discloses a continuous sheath of open-celled porous plastic, preferably PTFE, is used on the outside of a medical lead, extending along the lead body and the electrodes. Because the plastic is open-celled, when the pores are filled with saline, the lead can deliver electrical energy through the pores in the plastic. Pore size is chosen to discourage tissue ingrowth while allowing for defibrillation energy delivery and electrical signals through it. The porous plastic has a biocompatible wetting agent applied to it to speed the process of filling the pores with saline. The pores over nonelectrode regions may be filled with a nonconductive or conductive polymer to further prevent tissue ingrowth. Likewise, the conductive portions may have pores filled with a conductive polymer to further prevent tissue ingrowth, such as a hydrogel, for example, polyethylene oxide (PEO).
U.S. Pat. No. 5,919,570 describes a blend of hydrogel and polymer. Slippery, hydrophilic coating compositions of a polyurethane/urea prepolymer adduct intermediate commingled with at least one dissimilar hydrogel polymer precursor, and materials composed of a polymeric plastic or rubber substrate or a metallic substrate with a slippery hydrogel coating of a polyurethane/urea prepolymer adduct intermediate and at least one dissimilar hydrogel thereon, such that the coating composition tenaciously adheres to the substrate, are disclosed. The coating compositions and coated materials are non-toxic and biocompatible, and are ideally suited for use on medical devices, particularly, catheters, catheter balloons and stents. The coating compositions, coated materials and coated devices demonstrate low coefficients of friction in contact with body fluids, especially blood, as well as a high degree of wear permanence over prolonged use of the device. The hydrogel coatings are capable of being dried to facilitate storage of the devices to which they have been applied, and can be instantly reactivated for later use by exposure to water.
U.S. Pat. No. 5,902,329 describes an extractable lead and method for chronic blood contacting use. The new lead contains a hydrogel coating having a thickness increase greater than 10% when hydrated. A thick coating is used to provide a shear layer so that the coating tears during extraction, either at the coating/lead interface, between layers of the coating itself, or at the coating/tissue interface.
Many hydrogels are referred to in the art as superabsorbent polymers (SAP) which are generally polymeric materials containing water-insoluble long chain molecules with a low degree of cross-linking which are capable of forming hydrogel networks. In the presence of water or aqueous solutions such as body fluid, these hydrogel networks swell into a soft, resilient xe2x80x9cjelly-likexe2x80x9d material. When the swelling fluid is 0.9% saline, urine, or synthetic urine, these polymers may ultimately swell up to about 25-40 times their original weight. On the other hand, pulp fibers have a capacity to swell by a factor of only about 7-10 times by comparison.
The SAP materials are typically produced as granules that may then be mixed with pulp fibers during the formation of the absorbent core. Thus, with such highly absorbent granular material, it becomes possible to design and produce absorbent articles with roughly xc2xd to ⅓ of the bulkiness of the 100% pulp core. Reduction in volume of this nature is the subject of numerous U.S. Patents including for example,
U.S. Pat. Nos. 4,950,264 (Osborn); 4,467,012 (Pederson); and 4,217,901 (Bradstreet), In addition, processes for obtaining a hydrogel by cross-linking an acidic amino acid were reported by Akamatsu et al. in U.S. Pat. No. 3,948,863 (corres. JP Kokoku 52-41309) and Iwatsuki et al. in JP Kokai 5-279416. Further, use of cross-linked amino acid polymers as superabsorbent polymers was reported by Sikes et al. in JP PCT Kokai 6-506244 (corres. U.S. Pat. Nos. 5,247,068 and 5,284,936), Suzuki et al. in JP Kokai 7-309943 and Harada et al. in JP Kokai 8-59820. Superabsorbent polymers having high saline-absorbency are disclosed in JP Kokai 7-224163.
These types of devices may be inserted into a patient by a number of different medical procedures. The less invasive or traumatic the procedure, the more desirable is that procedure. For example, although the electrodes may be inserted by open chest surgery, the delivery of the electrode through catheterization techniques through arteries or veins is much more preferred. The difficulties involved with passing a sharp element through the vasculature of a patient can be readily appreciated, especially where the path can be tortuous or partially clogged with deposits. To avoid damage to the patient, the GUIDANT(trademark) Sweet-Tip(trademark) Model 4269 and the GUIDANT(trademark) Sweet Pico Tip bipolar endocardial leads provide a mannitol cap over the helical element in the lead. The mannitol cap provides a protective cover for the helical element which prevents the point of the helical element from scraping or puncturing interior walls of the vasculature or other tissue during introduction of the element to the patient. The mannitol effectively dissolves during the procedure, depending on the placement of the lead and other environmental factors, usually over the course of about 4 to 10 minutes. With certain lead designs and target locations, the leads can be inserted and used when there is only partial dissolution of the caps. This practice of providing caps on the leads has been effective in preventing damage to the patient during the introduction of the lead. Improper use of the lead, as by unauthorized immediate insertion, can lead to dislodgement or unsatisfactory pacing, which could occur with the misuse of any lead.
Other formats for delivering helical or barbed elements to secure an electrode into contact with appropriate tissue have utilized securing elements which are in a retracted position within the end of the delivered electrode. The retracted element is advanced into an exposed and operative position after positioning of the distal end of the electrode element within the patient. Advancement and exposure of the retracted element may be effected, for example, by winding or screwing a helical element through a hole in the most forward area of the electrode or by simply advancing a straight element through a hole.
There have been at least two areas identified by the present inventors where improvements may be made in the use of mannitol caps in the protection of helical leads or securing elements. Because of the physical shape of the helical element, mannitol present within the core of the helix tends to be dissolved out more slowly than desirable from within the helix and adjacent any electrode at the proximal end of the helical element. Additionally, any slowly dissolving mannitol that does remain within the confines or central area of the helix may have a tendency to slow down the advance of the helical element through the tissue until all of the mannitol in the core area has been removed. The lack of consistent rates of dissolution of the caps from the helical element, for example where the lead was prematurely positioned into soft tissue, tends to require surgeons to wait for a maximum length of time to provide assurance of the cap dissolution and proper electrical contact. Although neither of these considerations affect the in place performance of the connected leads, the reduction in procedural time by reducing or eliminating these effects is desired.
U. S. patent application Ser. No. 09/056,283 filed Apr. 7, 1998, now U.S. Pat. No. 6,091,978 describes helical connectors are provided with protective caps of an aqueous soluble or aqueous dispersible material wherein there is a hollow area or porous area of said material in a region within the enclosing region of the helix, with the aqueous soluble/dispersible materials including sugars such as mannitol. Although this construction significantly assists in the function of the removal of the cap, the beneficial effect is clearly dependent upon the ability to consistently shape and form specific designs in the cap structure. It is desirable to be able to effect dissolution parameters or rates that may be more generally controlled, as by selection of unique materials for use in the cap.
Caps for insertable leads, particularly for helical electrical connector leads comprise a blend or mixture of a first water-soluble or water-dispersible component (e.g., sugar(s) such as mannitol) and a hydrogel. The combination of the two materials provides increased control over dissolution/dispersion rates and the physical strength and characteristics of the cap. The cap material may be solid or may be constructed with specific structural characteristics (hollow, porous, external shaping such as grooves, core shell construction, etc.) to provide a range of physical characteristics.